1. Field of the Invention
The present invention relates to a load control device for controlling the amount of power delivered from an alternating-current (AC) source to an electrical load. In particular, the present invention relates to a load control device for receiving a first control signal having a first magnitude and for generating a second control signal having a second magnitude substantially greater than the first magnitude.
2. Description of the Related Art
A conventional wall-mounted load control device is mounted to a standard electrical wall box and is coupled between a source of alternating-current (AC) power (typically 50 or 60 Hz line voltage AC mains) and an electrical load. Standard load control devices, such as dimmers and motor speed controls, use one or more semiconductor switches, such as triacs or field effect transistors (FETs), coupled in series between the source and the lighting load to control the power delivered to the load, and thus, the intensity of the lighting load or the speed of the motor. Using a phase-control dimming technique, the dimmer renders the semiconductor switch conductive for a portion of each line half-cycle to provide power to the lighting load, and renders the semiconductor switch non-conductive for the other portion of the line half-cycle to disconnect power from the load.
Often, wall-mounted load control devices are coupled to a digital communication link for transmitting and receiving digital control signals. For example, a GRAFIK Eye® Control Unit, manufactured by the assignee of the present invention, includes a plurality of dimming circuits for control of a plurality of lighting loads and is operable to be coupled to a remote control wallstation via a low-voltage digital communication link. The GRAFIK Eye Control Unit is operable to control each of the plurality of lighting loads in response to the digital control signals received from the remote control wallstation. The GRAFIK Eye Control Unit is described in greater detail in commonly-assigned U.S. Pat. No. 5,430,356, issued Jul. 4, 1995, entitled PROGRAMMABLE LIGHTING CONTROL SYSTEM WITH NORMALIZED DIMMING FOR DIFFERENT LIGHT SOURCES, the entire disclosure of which is hereby incorporated by reference.
The National Electrical Code (NEC) provides guidelines for the safe installation of electrical equipment and wiring. The National Electrical Code (2005 Edition) defines a “Class 2” circuit as “the portion of the wiring system between the load side of a Class 2 power source and the connected equipment”. Class 2 AC circuits are generally limited to 42.4 volts peak and 100 VA. Class 2 direct-current (DC) circuits are generally limited to 60 volts and 100 VA. Class 2 circuits include, for example, thermostats, burglary and security systems, cables (twisted-pair or coaxial) that interconnect computers for Local Area Networks, and limited-energy voice, intercom, and sound systems. Low-voltage digital communication links coupled to lighting control devices, e.g., the GRAFIK Eye Control Units, are typically classified as Class 2 circuits.
Article 725.55 of the National Electrical Code (2005 Edition) states that Class 2 circuits may not be placed in the same enclosure as the conductors of electrical light and power wiring, except when necessary to connect to equipment utilizing the Class 2 circuit and the electrical light and power wiring is kept physically separated by a minimum of 6 mm (0.250″) or operates at 150 volts or less to ground and the Class 2 circuits are installed appropriately with the proper grade of Class 3 cable. Since typical electrical loads and typical load control devices operate at 120 VRMS in the United States, the GRAFIK Eye Control Unit can be coupled to both a 120-VRMS power source and a Class 2 digital communication link and be installed in an electrical wallbox and still satisfy the National Electrical Code.
However, many electrical loads, such as electronic ballasts, require a higher voltage to operate, e.g., 277 VRMS. According to the National Electrical Code, the GRAFIK Eye Control Unit cannot be coupled to both a 277 VRMS power source and a Class 2 digital communication link and be installed in a single electrical wallbox. Therefore, there is a need for a load control device for controlling a 277-VAC ballast in response to a digital control signal, where the load control device can be installed in a single wallbox such that the installation meets the requirements of the National Electrical Code.